This invention relates to an exposure process for use in a variable magnification copying machine.
A copying technique is well known utilizing a drum-, belt- or sheet-shaped photosensitive member for electrophotography, which is subject to a slitwise exposure to form an electrostatic latent image thereon. Generally speaking, a slitwise exposure technique utilizes a slit-shaped illumination to scan an original so that an image of the illumination which is formed by an imaging lens system scans a photosensitive member for purpose of exposure, and thus is achieved by a relative movement between an original, an exposure optics and the photosensitive member. Conventional arrangements to achieve such a slitwise exposure may comprise a relative movement between a set including an original and a photosensitive member and an exposure optic system, or may comprise a pair of plane mirrors disposed on the object side of an imaging lens system for movement in order to scan an original slitwise, whereby an image of an original area being scanned may be focussed on a given location within the space of the copying machine which defines an exposure station, across which the surface of a photosensitive member is moved.
FIG. 1 illustrates an arrangement in which an original 0 placed on an original receptacle 1 is slitwise scanned by a lamp 2 in a direction indicated by an arrow, while simultaneously moving an imaging lens system 3 from a solid line position to a broken line position, thus focussing an image of an area of the original which is being illuminated onto an exposure station P, across which the surface of a photosensitive member 4 is moved. FIG. 2 illustrates another technique in which an imaging lens 3 is fixedly disposed while a pair of plane mirrors 6, 7, which are disposed on the image side of the imaging lens, are moved either integrally or separately as the original is slitwise scanned by an illumination from the lamp 2, thereby forming an image of an area of the original being illuminated onto an exposure station P, across which the surface of the photosensitive member 4 is moved. It is to be understood that for purpose of simplicity of description, similar parts are designated by like reference numerals in both FIGS. 1 and 2. In these figures, numeral 5 represents a plate adapted to cut off part of imaging light flux in order to define the slit width of the exposure station, but this will be referred to hereinafter as a slit plate in order to distinguish it from a light shield member to be described later.
It will be seen in FIGS. 1 and 2 that the original 0 is illuminated by the lamp 2, and the resulting light image is utilized for purpose of exposure. The configuration of the exposure station is defined to be a slit form by the slit plate 5. As viewed in FIGS. 1 and 2, the lengthwise direction of the slit-shaped exposure station P will be defined as a direction which is perpendicular to the plane of the drawing. It will be seen that the surface of the photosensitive member 4 moves in a direction which is perpendicular to the lengthwise direction of the exposure station P. However, considering the relationship between the surface of the photosensitive member 4 and the surface of the receptacle on which the original is placed, it will be seen that the surface of the photosensitive member moves in a plane parallel to the surface of the receptacle. The direction in which the surface of the photosensitive member moves at the exposure station P will be referred to hereinafter as a slitwise scan direction, which is the horizontal direction, as viewed in FIGS. 1 and 2. In FIGS. 1 and 2, the angle of incidence of exposure light flux which impinges upon the imaging lens system 3 continuously varies in the slitwise scan direction as the slitwise exposure proceeds. In the arrangement of FIG. 1, it will be seen that the imaging lens system moves in the slitwise scan direction or in a direction perpendicular to the optical axis thereof.
The illumination of the original 0 may take place by a slitwise scanning as mentioned above, or the entire surface may be concurrently illuminated.
It is a prerequisite to form a satisfactory electrostatic latent image on the photosensitive member that the surface of the photosensitive member be uniformly exposed throughout. For a slitwise exposure, this means that the distribution of exposure light be uniform lengthwise of the slit-shaped exposure station.
When the aperture efficiency and the biquadratic law of cosine in the imaging lens system are taken into consideration, it is necessary to achieve a uniform distribution of exposure light in the manner mentioned above that the distribution of the intensity of illuminating light incident on the original receptacle be such that the intensity is low at the middle and is high at the opposite ends, as viewed in the lengthwise direction of the exposure station or in a direction perpendicular to the plane of the drawing of FIGS. 1 and 2. At this end, the illumination of the original is performed in a manner to satisfy such requirement.
This presents no problem whatsoever where the copying machine uses a fixed magnification. However, in a variable magnification copying machine in which the imaging lens system is displaced in accordance with a particular magnitude of the magnification being utilized, a uniform distribution of exposure light over the lengthwise direction of the exposure station which may be achieved at a particular magnitude of copying magnification cannot be maintained when the magnification is changed.
It will be appropriate to mention the displacement of the imaging lens system here in some detail as the magnitude of the copying magnification is changed. It should be noted such displacement includes a center referenced displacement and a displacement from an offset reference. Specifically, referring to FIGS. 3 and 4, numeral 1-1 represents the surface of the original receptacle and numeral 4-1 the surface of the photosensitive member. As viewed in these figures, the horizontal or left-to-right direction corresponds to the lengthwise direction of the exposure station. Points A, B, C and D represent the ends of effective copying regions on the surfaces 1-1 and 4-1. When the imaging lens system 3 assumes the solid line position, the magnification is unity, and the points A and B correspond to points C and D, respectively. When the imaging lens system 3 assumes a phantom line position, the copying magnification is less than unity or the image is focussed on a reduced scale. For the displacement of FIG. 3, the points A and B correspond to points A' and B', respectively, on the surface of photosensitive member 4-1. However, a middle point q between the points A and B on the receptacle surface 1-1 corresponds to a middle point q' between the points C and D on the surface of photosensitive member 4-1. A displacement of a focussing lens system which yields such correspondence is referred to as a center referenced displacement. On the other hand, in the arrangement of FIG. 4 when using a reduced magnification, the points B corresponds to a point B', but a point A' which corresponds to the point A coincides with the point C which is produced when the magnification of unity is used. A displacement of the imaging lens system which yields such correspondence is referred to as an offset reference displacement. The same applies for a magnification greater than unity as that described above in connection with a magnification less than unity. It is to be understood in the description of FIGS. 3 and 4 that a change in the path length between the surfaces 1-1 and 4-1 which is generally required as the magnification is changed is neglected.
Returning to distribution of exposure light mentioned above, it is to be noted that for the arrangement of FIG. 3, if the distribution of the intensity of light illuminating the original is properly set for a magnification of unity, the distribution of exposure light as viewed in the lengthwise direction of the exposure station will be changed for a magnification less than unity such that the exposure light will be excessively high toward the points A' and B' as compared to the point q'. Similarly, for the arrangement of FIG. 4, the exposure light will be excessively high toward the point B' in a progressive manner. FIG. 5 illustrates a technique which compensates for the distribution of exposure light for a magnification less than unity in the arrangement of FIG. 4. Specifically, when a magnification less than unity is utilized, the slit plate 5 is displaced in a manner shown by sold line in FIG. 5 so that the width of the slit defined in the exposure station progressively narrows from the point C toward the point B'. In FIG. 5, the broken lines indicate the position of the slit plate assumed when the magnification of unity is used.
The described technique is effective to a degree for a slit width of the exposure station which is equal to or greater than 15 mm, but is hardly applicable to an arrangement utilizing a slit width on the order of 5 mm which is recently frequently employed in order to achieve an image of a high quality. Also where the intensity of exposure light is not uniform crosswise of the slit, as exemplified by a centrally topped distribution, the described compensation technique cannot achieve a uniform distribution of exposure light. Furthermore, it is practically infeasible to correct the distribution of the intensity of light illuminating the original each time the magnification is changed.